1. Field
The present invention relates generally to wireless communications, and more specifically to inter-frequency hard handoff of a wireless access terminal.
2. Background
Since the Internet explosion in the late 1990's, the market for wireless Internet applications has grown rapidly. As wireless communication systems have evolved from the first voice-only cellular telephone systems to high-speed digital data networks capable of delivering voice, Internet, and even streaming video content to miniature handheld units, so have the applications that rely on wireless networks. Wireless communication systems now include high-speed digital data networks capable of delivering voice, Internet, and even streaming video content to handheld units that users carry with them and use as they travel. Several systems have been proposed to satisfy the growing need for efficient, high-throughput packet data services.
Wireless packet data systems have been designed to allow users to access packet data networks such as the Internet from stationary or mobile user equipment. The user equipment is sometimes called an Access Terminal (AT), which connects wirelessly with the Internet through a wireless infrastructure system called an Access Network (AN). The AN generally includes multiple Access Points (AP), each having a limited transmit and receive range. The area within the transmit and receive range of a particular AP is a coverage area.
A wireless data system that supports non-stationary ATs must permit an AT to move from the coverage area of one AP to that of another AP without losing the AT's connection with the AN. The process of rerouting data for an AT from a first AP to a second AP is called handoff. This rerouting process generally causes a brief interruption in data communication with the AT. A handoff of an AT from one AP to another AP may require changing channels within the same frequency band (intra-frequency handoff) or may require changing frequency bands (inter-frequency handoff). The TIA-856-A standard, entitled “cdma2000 High Rate Packet Data Air Interface Specification,” and published by the Telecommunications Industry Association in 2004, describes methods of minimizing the duration of the interruption of data communication during intra-frequency handoff of an AT. But TIA-856-A does not provide ways to perform seamless inter-frequency handoff of an AT. Handoff of an AT from a source frequency band F1 to a target frequency band F2 will thus cause unacceptably long interruptions to services such as Voice over IP (VoIP), video telephony, network gaming, or other applications requiring high Quality of Service (QoS). Accordingly, it would be desirable to minimize the interruption in data flow that occurs during inter-frequency handoffs of an AT.